We have shown that pterin carbinolamine dehydratase (PCD) is a dual function protein. In the cytoplasm of the cell, it functions as an essential component of the phenylalanine hydroxylating system that serves to convert phenylalanine to tyrosine. PCD, together with another enzyme, dihydropteridine reductase, functions to regenerate tetrahydrobiopterin (BH4), the essential coenzyme for the hydroxylating system. In the nucleus of the cell, PCD, plays a totally different role: it combines with and stabilizes HNF-1, a gene transcription factor, thereby enhancing the transcriptional activity of this protein. In this role, PCD has become known as DCoH. HNF-1 is involved in the transcription of a whole family of hepatic genes including those coding for proteins such as serum albumin of alpha-fibrocogen and pyruvate kinase. Studies with HNF-1 knockout mice have shown, surprisingly, that phenylalanine hydroxylase is one of the proteins whose synthesis is under the control of HNF 1. These findings raised the possibility that PCD/DCoH might be involved in regulating the synthesis of phenylalanine hydroxylase. We have shown that the 5'-flanking region of the human phenylalanine hydroxylase gene contains two sequences that bind HNF-1. When plasmids expressing HNF-1 and PCD/DCoH were transfected into Chinese hamster ovary cells together with a plasmid containing the appropriate 5' flanking region of the phenylalanine hydroxylase gene, the expression of the phenylalanine hydroxylase gene was stimulated 8-fold by the HNF plasmid and a further 1.6- fold by PCD/DCoH. These results show that PCD/DCoH do indeed play a role in activating the expression of phenylalanine hydroxylase. We plan to study whether other components of the phenylalanine hydroxylating system such as phenylalanine and BH4 can effect the ability of HNF-1 and PCD/DCoH to stimulate the expression of phenylalanine hydroxylase. Phenylalanine hydroxylase is more abundant in livers of diabetic rats. This finding is in accord with other observations showing that there is an increase in the activity of phenylalanine hydroxylase under conditions favoring gluconeogenesis. We have now shown that the converse is true, i.e., that providing the animal with alternative dietary gluconeogenic substrates such as glycerol or fructose leads to a decrease in the activity of phenylalanine hydroxylase. This decrease appears to be secondary to a decreased concentration of BH4 which is due to a decrease in the concentration of GTP, the precursor of BH4. Given the importance of peripheral gluconeogenesis in providing the brain with its essential fuel, these aspects of hepatic phenylalanine metabolism can affect the development and functioning of the brain.